1. Technical Field
The present invention relates generally to exhaust systems and more particularly to a system for mixing ambient air with exhaust air to reduce exhaust gas temperature.
2. Description of the Problem
Elevated exhaust gas temperatures from the tail pipes of diesel engine equipped motor vehicles have become a greater problem with the introduction of diesel particulate trap/filters (DPF) and the need for regeneration of these filters. Diesel particulate filters remove from the exhaust flow of diesel engines unburned and partially burned hydrocarbons (a major part of the particulate matter produced by diesels during combustion). A DPF may be periodically regenerated by raising the temperature in the filter sufficiently to oxidize the particulate matter. Doing so increases the temperature of the exhaust to a degree that it may harm passers by where the exhaust is discharged near ground level.
Exhaust system cooling using a venturi in the exhaust to draw in cooling air through vents is known from U.S. Pat. No. 4,265,332. In the '332 patent a sleeve (cylindrical heat shield) with an open upstream end was placed around a portion of an exhaust pipe including the muffler. The portion of the exhaust pipe extending from the muffler terminated in constricted nozzle. The sleeve extended past the constricted nozzle/venturi to become, in effect, an extension of the exhaust pipe. The venturi accelerated the exhaust drawing cooling air in through the open end of the sleeve upstream of the muffler, around the exhaust pipe and eventually into the exhaust stream. Though this is done to cool the exhaust system, particularly the muffler, the air is mixed with the exhaust before final discharge to atmosphere. The venturi and sleeve were fixed elements of the exhaust system assembly.